The present invention relates to a method for controlling the selection of the gear reduction ratio of a transmission for a vehicle equipped with a stepped-ratio or continuously variable automatic transmission, especially for operation of the said vehicle with a speed governor or limiter. It is known that specific strategies can be used to control the selection of the transmission ratio of a stepped-ratio automatic transmission in order to suppress the instabilities of automatic gearboxes known as the hunting phenomenon. This phenomenon is manifested by an excessive frequency of ratio changes from one ratio to a higher ratio and vice versa, when the resisting load exerted on the vehicle becomes large (inclined roadway, strong wind, etc.). In fact, when the resisting load increases, as in the case when the vehicle encounters an uphill stretch, the said vehicle tends to slow down, and the operator presses on the accelerator pedal in order to compensate for this loss of speed. When the operator's demand (depression of the accelerator pedal) exceeds a certain threshold, the automatic management system of the gearbox initiates a downshift to a shorter ratio. This downshift permits the vehicle to deliver sufficient thrust to accelerate, and the operator tends to raise his foot from the accelerator pedal once the desired speed has been reached.
Once the vehicle has regained its initial speed and the operator has raised his foot from the accelerator pedal, the automatic management system of the gearbox initiates a shift to the higher ratio, re-establishing the initial conditions that are going to lead to a loss of vehicle speed (since the resisting load has not decreased). Once again, the operator will attempt to compensate for this loss of speed, a shift to the lower ratio will be initiated and so on . . . This phenomenon of oscillation between two gear reduction ratios of the transmission also occurs when the vehicle speed is controlled by an automatic speed governor, the said governor exerting pilot control over the same control means as the operator (degree of opening of the intake butterfly valve, for example, in the case of controlled ignition engines) to perform its speed-governing function.
In order to eliminate this phenomenon of ratio hunting, there have been proposed, especially in connection with automatic speed-governing systems, solutions to take control of the automatic management system of the gearbox, and in particular to force downshifts or to prevent lengthening of ratios as a function of specific criteria. For example, U.S. Pat. Nos. 4,421,192 and 4,697,478, European Patent 142046 and U.S. Pat. Nos. 5,270,934 and 5,479,349 describe systems for governing the speed of a vehicle by using, for pilot control of stepped-ratio automatic transmissions, algorithms written specifically to eliminate the hunting phenomenon. These algorithms lead to forced initiation of a downshift (shift to a lower ratio) upon detection of a loss of vehicle speed compared with a given setpoint. Depending on the case, following such a forced downshift, the method consists in forcing the vehicle to remain in the lower ratio for a certain time after the vehicle has regained its setpoint speed (U.S. Pat. Nos. 4,421,192 and 4,697,478), or in making the shift to the higher ratio contingent upon certain conditions. In the case of U.S. Pat. No. 5,479,349, the shift to the higher ratio is prevented as long as the absolute value of the speed difference and that of the current acceleration exceed predetermined values. In European Patent 142046, the lower ratio is maintained as long as the vehicle speed cannot be sustained without opening the air-intake butterfly valve beyond a predetermined limit. In U.S. Pat. No. 5,270,934, the condition for remaining in the lower ratio involves a predictive estimate, based on the control signal to open the butterfly valve, of the vehicle speed if the higher ratio were engaged.
The solutions for eliminating the hunting problem, as proposed in the aforesaid documents, are limited to the context of use of a speed-governing system, and they are not capable of anticipating sufficiently to avoid speed fluctuations during an increase of the resisting load exerted on the vehicle, downshifting being initiated only after detection of a substantial speed loss.
In addition, the case of use of a speed limiter poses a new problem compared with the prior art cited in the foregoing, in the sense that a speed limiter tends to function in a more extensive range of speed and ratios (approaching low speeds and low ratios) than does the speed governor, used mainly on the highway at high speed and at the highest transmission ratios.
U.S. Pat. Nos. 4,709,595 and 5,012,419 propose contrivances to attenuate the jolt associated with downshifting, but do not offer a solution to the need to anticipate the downshift.
U.S. Pat. No. 5,241,476, French Patent 2737761 and U.S. Pat. No. 5,738,605 describe techniques for elimination of the hunting problem based on inhibition of the shift to a higher ratio as a function of criteria based on a predictive estimate of the acceleration that the vehicle would experience at the higher ratio. These latter three documents certainly teach us elaborate methods for estimating the available acceleration at the higher ratio with good precision, but their teachings are limited to a strategy of inhibiting the shift to the higher ratio to eliminate the hunting problem, without giving any indication with regard to initiating the downshift.
For implementation of the methods cited in the foregoing, it is necessary to provide pilot-control means for inhibiting/authorizing the shift to a higher ratio or for forcing a downshift to the lower ratio. Solutions are proposed with regard to achieving such pilot-control means in connection with stepped-ratio transmissions.